In the formation of synthetic resin foils or films, the extruded thermoplastic tube is blown to decrease the wall thickness and transform the extruded strand into a tube of a foil or film thickness, this tube being flattened to produce a two-layer web or, where a number of tubes are formed one within another, a multi-layer web. (See, for example, the commonly assigned U.S. Pat. Nos. 3,726,743, 3,926,706 and 3,957,566).
Generally, the film or foil blowing machine is also provided with a coiling unit on which the flattened web is coiled for storage of the foil, for subsequent handling, etc.
The coils can then be delivered to, for example, a machine for making bags, to devices for printing or cutting the web, or to packaging apparatuses in which the web is utilized to envelop an article to be packaged.
In general, the head at which the extruded strand of the thermoplastic material emerges to be blown to form the foil tube can be considered to be a flaw-producing member since thickness flaws frequently develop at the point of emergence from this member and can be propagated over the length of the blown tube.
The tube is blown at a substantially constant rate and hence the flattened foil web, consisting of at least two layers for the reasons described, emerges at a substantially constant rate from the foil blowing apparatus and is delivered to the coiling device.
So that any thickness defect may not be concentrated at a single point along the coil or along the web, the member at which the foil tube emerges from the blowing head is generally rotated or angularly oscillated so that any thickness defect will generally be laid down in a more or less periodic pattern. The pattern is determined by the angular velocity of this member. When the web is coiled, the defect pattern lies along an arc whose radius is a function of the product of the angular velocity of the member, the radius of this member and the coiling time for the particular layer.
The web, which can be flattened from the tube, can have at least two layers which can be connected along opposite longitudinal edges of the flattened tube or only along one edge if, for example, the tube is longitudinally slit.
The angularly displaceable member, which determines the point at which the thickness defect may be produced in the foil tube, can be the injection head for the extrusion of the thermoplastic tube, a ring against which this tube expands or a like member (see Schenkel, Kunststoff-Extrudertechnik, Muinch, 1963, pages 390-393) or other means for laying down the foil including bars or the like to which a similar periodic movement is imparted.
In spite of the fact that an angular displacement is important to the member to prevent concentration of thickness defects in a single longitudinal zone of the web or the tube, problems are encountered in the prior art devices. For example, it is common to maintain this angular velocity substantially constant and at a speed chosen to permit the flattening of the foil tube in such manner that it is free from folds.
It is customary to operate at the highest possible angular velocity which will not result in the formation of folds.
This system largely prevents thickness defects from being additive or cumulative on coiling and prevents the formation of bulges or ridges in one circular zone of the coil.
However, in practice it is found that the arc over which any defect is laid don in the coil increases in radius from layer to layer and, while a severe bulge at one location may not result, there is nevertheless a tendency for the coil to depart from a cylindrical configuration and assume a conical or other distorted configuration, i.e. the configuration of a one-sheet hyperboloid of revolution.
Such noncylindrical coils are difficult to handle in subsequent processing machinery, for example, bag manufacturing machines. The problem is especially pronounced when tools or guide members must back directly against the coil or in a system in which the periphery of the coil determines other operating parameters of the machine.